The object of the present invention is a process for homologating alcohols through the intermediate production of ketals with cobalt carbonyl complexes as catalysts.
The alcohol homologation producing, by reaction with carbon monoxide and hydrogen, the alcohol and/or aldehyde with one additional carbon atom, can be written: EQU ROH+CO+2H.sub.2 .fwdarw.R-CH.sub.2 OH+H.sub.2 O
and/or EQU ROH+CO+H.sub.2 .fwdarw.R-CHO+H.sub.2 O
This reaction has long been known and is usually catalyzed by cobalt carbonyl complexes, such as, for example, dicobalt octacarbonyl. It is however necessary, for obtaining a reasonably fast conversion, to operate under severe temperature and pressure conditions, and by-products are formed in substantial amounts.
It has been proposed to add various promoters to the catalyst, in order to improve its activity. Among them, iodine and iodine compounds occupy a major place. There can be used an organic iodine compound, such as an alkyl iodide, or an inorganic iodine derivative, such as an alkali metal iodide. Recently the joint use of an organic iodide and an inorganic iodide has been claimed to improve the conversion of methanol to ethyl compounds, ethanol and/or acetaldehyde.
However the iodine derivatives have the disadvantage, in addition to their cost which makes their recycling necessary, to leave traces of iodine in the products which are thus polluted and even sometimes made unsuitable for certain purposes. This is, for example, the case of 2-phenyl ethanol, which could be prepared by homologation of benzyl alcohol but which must be made completely free of iodine for use in the perfume industry.
The homologation of acetals by carbonylation has also been described in the literature (I. Wender and P. Pino, Organic Syntheses via Metal Carbonyls, vol-2, J. Wiley, 1977, pages 26 to 32), but the reacting portion is the aldehyde portion of the acetal, in any case under very severe conditions, and not the alcohol portion.
The carbonylation of orthoesters (Wender and Pino, already mentioned) leads conversely, under mild conditions (100.degree. C., 150 bars of H.sub.2 +CO synthesis gas) to the homologation of the alcohol portion of the molecule, which could thus contitute an alternative way to homologate an alcohol. Unhappily, the synthesis of orthoesters is difficult and requires costly and difficultly available reactants.
It has been found, and this constitutes the object of the invention, that the carbonylation of ketals results in the homologation of the alcohol portion of the ketal with high yields under mild conditions. The main product is the acetal of the homologous aldehyde in admixture with a low proportion of free homologous aldehyde. This acetal can be easily hydrolysed to aldehyde. The reaction is much faster than the homologation of the corresponding alcohol under the same conditions, and the selectivity to homologous products is higher.